Backstories can potentially kill a build because the more character details you explain the more choices you remove from anyone wanting to play it...
You have to provide enough information to make the character interesting without removing options for a player.
The funniest backstories I've seen are for stealth classes...
"This character is an absolute shadow, nobody sees him coming until they feel the sharp tip of his penis in their back......
......he got caught crossing the border, and so his story begins"